CHICAGO (CBS) — Something that happened over downtown Chicago Thursday morning is confirmation that the fear of another 9/11 is never far from people’s minds.

As WBBM Newsradio’s Steve Miller reports, two blocks away from the Willis Tower, Tracy McBride had a window seat in her office as she watched a plane come in low in the clear blue sky.

“Saw a plane coming literally right over the Loop. And I first said, something’s not right. That’s scary. And a few of us stood up. We looked out the window, and it literally looked like the plane was going to hit the tower,” McBride said.

LISTEN: WBBM Newsradio’s Steve Miller reports

She said one thought went through their minds.

“9/11. How frightening it was. It was a sickening feeling; literally, a nauseating feeling,” she said.

McBride said she’s worked in that office near the Willis Tower for five years and looks out the window everyday, so she’d notice something unusual in the sky.

“I’m right there all day and I’ve never seen something come that close to the Loop before, it was frightening,” she said.

FAA Spokeswoman Elizabeth Isham Cory said, “We did know about a small aircraft that did have permission to fly downtown today for possibly a photo mission.”

Cory said planes generally cannot be less than 1,000 feet above downtown buildings, but if a plane does need to fly that low, it must have advance permission.

The plane that flew near the Willis Tower Thursday morning would have needed that permission. It also could have been a law enforcement mission.

An FBI spokesman said the FBI flies missions “almost daily,” but he would not confirm that this was an FBI plane flying low over the Loop.